This invention relates generally to strip line and microstrip electronic circuits, and more particularly the invention relates to a balun for use in such circuits.
The balun is a device for matching an unbalanced coaxial transmission line with a balanced two wire system. In its simplest form, the device provides a low reflection coefficient transition between the balanced transmission line and the unbalanced line. In an unbalanced line, one of the two transmission line conductors is at ground or zero potential. A balanced line is defined as one in which the voltage to ground of the two transmission lines are equal and in opposite phase.
FIG. 1 is an illustration of a typical coaxial balun in which an unbalanced input is connected to a center conductor 10 of a coaxial transmission line with the outer conductor 12 grounded at the input. A balanced output is taken at an opposite end of the coaxial transmission line from the center conductor and the outer conductor as illustrated. While such a coaxial balun produces good results over a wide frequency bandwidth, the geometry does not lend itself well to modern surface mount technology.
Radio frequency circuits are typically embodied in hybrid circuits in which active and passive circuit components are mounted on a surface of an insulative substrate such as a ceramic substrate with the components interconnected by printed metallic conductors of copper, gold, or tantalum, for example. In the microwave range, the conductors become transmission lines. Stripline and microstrip transmission lines embody a conductor in one or more spaced ground planes.
Heretofore, discrete coaxial baluns have been mounted on a hybrid circuit substrate and interconnected with other circuit components by leads. This increases assembly cost in cutting and installing the coaxial balun.
Attempts have been made to replace coaxial baluns in hybrid circuits by using microstrip conductors. The present invention is directed to an improved microstrip balun especially suitable for surface mount technology.